This invention relates to a method and an apparatus for transmitting digitized voice signals, in a telecommunication environment in which the compression and decompression of voice signals is involved. More specifically, it relates to a method and an apparatus for improving the quality of an audio signal, which has been compressed or encoded with a digital signal processing technique, when the signal is stored or retrieved to or from a store-and-forward (e.g. voicemail) system in a telecommunication network.
In recent years, the telecommunications industry has witnessed the proliferation of a variety of digital vocoders in order to meet bandwidth demands of different wireline and wireless communication systems. Vocoders are usually integrated in wireless telephones and the base stations of the communication network or a link therefrom. They provide speech compression of a digitized voice signal as well as the reverse transformation. Typically, a voice signal is digitized through one of many quantization techniques. An example of this technique is Pulse Code Modulation (PCM). For the purposes of this description, we will refer to PCM as the input format for the vocoder. Thus a vocoder includes an encoder stage that will accept as input a digitized voice signal and that will output a compressed signal, a possible compression ratio being 8:1. As for the reverse transformation the vocoder is provided with a decoder stage that will accept the compressed speech signal and that will output a digitized signal, such as PCM samples.
The main advantage of compressing speech is that it uses less of the limited available channel bandwidth for transmission. In voice messaging applications, it also minimizes storage memory requirements thereby increasing the number of subscribers that can be served. The main disadvantage is loss of speech quality specially when speech is subjected to multiple instances of vocoders.
The rapid growth in the diversity of networks and the number of users of such networks is increasing the number of instances where two vocoders are placed in tandem to serve a single connection. An example of such a situation is when a wireless user records a message to a voice storage and forwarding device and a wireline terminal then retrieves the message. In such a case, a first encoder is used to compress the speech at the terminal of the wireless user. The compressed speech is transmitted to a base station serving the local wireless terminal where it is decompressed (converted to PCM format samples). The resulting PCM samples are routed to the voice storage and forwarding device where a second encoder is used to compress the input signal for storage in a database. If the person who has access to the voice storage and forwarding device retrieves the message from a wireline terminal, a speech decoder in the voice storage and forwarding device decompresses the stored compressed speech data into PCM format for transmission to the wireline terminal over the PSTN.
The situation is even more complex when the user uses a wireless terminal to retrieve, from a voice storage and forwarding device, a message that was recorded by a wireless terminal user. In such a situation, the compression and decompression of speech occurs three times: once for the wireless terminal/base station combination, a second time in storing and retrieving the message in the voice storage and forwarding device, and finally, a third time in the last base station/wireless terminal combination. Yet another expanding area of concern is Internet telephony. Indeed, Internet telephony uses vocoders as well.
In an attempt to eliminate the condition of vocoder tandeming, a method called  less than  less than bypass greater than  greater than  has been proposed in the past. The basic idea behind this approach is the provision of a digital signal processor including a vocoder and a bypass mechanism that is invoked when the incoming signal is in a format compatible with the vocoder. In use, the digital signal processor associated with the first base station that receives the RF signal from a first wireless terminal determines, through signaling and control that an identical digital signal processor exists at the second base station associated with the wireless terminal at which the call is directed. The digital signal processor associated with the first base station rather than converting the compressed speech signals into PCM samples invokes the bypass mechanism and outputs the compressed speech in the transport network. The compressed speech signal, when arriving at the digital signal processor associated with the second base station is routed such as to bypass the local vocoder. Decompression of the signal occurs only at the second wireless terminal. The xe2x80x9cbypassxe2x80x9d approach is described in the international application serial number PCT/95CA/00704 dated Dec. 13, 1995. The contents of this disclosure are incorporated herein by reference.
However, the xe2x80x9cbypassxe2x80x9d solution described above was designed for the base station portion of the telecommunication network. This implies that it is useful only for the tandem situations in which a telecommunication link is established between two wireless users. Also, this solution is only valid for identical vocoders. With the diversity of vocoders quickly increasing, the bypass solution is only valid for a small portion of connections involving tandem vocoding.
Thus, there exists a need in the industry for devices capable of improving voice quality in other situations that involve vocoder tandeming such as in digital voice store-and-forward systems.
An object of the invention is to provide an apparatus for storage and forwarding of voice data that allows to reduce signal degradations as a result of successive signal compression/decompression cycles, particularly when non-compatible vocoders are used to effect the compression/decompression cycles.
Another object of the invention is to provide a method for storage and forwarding of voice data that allows to reduce signal degradations as a result of successive signal compression/decompression cycles, particularly when non-compatible vocoders are used to effect the compression/decompression cycles.
Another object of the invention is a communication system with voice storage and forwarding capability, designed to reduce signal degradations as a result of successive signal compression/decompression cycles, particularly when non-compatible vocoders are used to effect the compression/decompression cycles.
As embodied and broadly described herein, the invention provides a voice storage and forwarding device, comprising:
an input for receiving a digital signal that conveys audio information;
a group of vocoders;
a switch capable of acquiring a plurality of vocoder selection positions, in each vocoder selection position said switch directing the digital signal that conveys audio information received at said input to a selected one of said vocoders of said group of vocoders;
a storage medium in a data communicative relationship with said vocoders, said storage medium capable to store data that conveys audio information issued by a vocoder from said group of vocoders and transmit stored data that conveys audio information toward a selected one of said vocoders.
In this specification, the term xe2x80x9cwireless terminalxe2x80x9d is intended to include both mobile terminals and fixed wireless terminals. The term xe2x80x9cwireless terminalxe2x80x9d is part of a larger family of terminals that we call xe2x80x9cspeech compression terminalsxe2x80x9d. These terminals usually include vocoders that are capable of converting speech from a digitized format to a compressed format and vice versa. Other examples of these terminals are those used for Internet telecommunications, Integrated Services Digital Network (ISDN) terminals, etc. This disclosure is therefore meant to include all terminals capable of converting speech from a digitized format to a compressed format and vice versa.
The expression xe2x80x9cdata framexe2x80x9d will refer to a group of bits organized in a certain structure or frame that conveys some information. Typically, a data frame when representing a segment of audio signal in compressed form will include a coefficients segment and an excitation segment. The data frame may also include additional elements that may be necessary for the intended application.
The expressions xe2x80x9cfirst formatxe2x80x9d, xe2x80x9csecond formatxe2x80x9d, etc. when used to describe the audio signal in compressed form in the format of a given vocoder, refers to signals in compressed form that are, generally speaking, not compatible with each other, although they may share a common basic structure. For example, such signals may be divided into a coefficient segment and an excitation segment. Thus, a vocoder capable of converting a signal under the first format will not, generally speaking, be capable of processing a signal expressed under any other format than the first format.
In this specification, the term xe2x80x9ccoefficient segmentxe2x80x9d is intended to refer to any set of coefficients that uniquely defines a filter function which models the human vocal tract. It also refers to any type of information format from which the coefficients may indirectly be extracted. In conventional vocoders, several different types of coefficients are known, including reflection coefficients, arcsines of the reflection coefficients, line spectrum pairs, log area ratios, among others. These different types of coefficients are usually related by mathematical transformations and have different properties that suit them to different applications. Thus, the term xe2x80x9ccoefficient segmentxe2x80x9d is intended to encompass any of these types of coefficients.
The xe2x80x9cexcitation segmentxe2x80x9d can be defined as information that needs to be combined with the coefficients segment in order to provide a complete representation of the audio signal. It also refers to any type of information format from which the excitation may indirectly be extracted. The excitation segment complements the coefficients segment when synthesizing the signal to obtain a signal in a non-compressed form such as in PCM sample representations. Such excitation segment may include parametric information describing the periodicity of the speech signal, an excitation signal as computed by the encoder of a vocoder, speech framing control information to ensure synchronous framing in the decoder associated with the remote vocoder, pitch periods, pitch lags, gains and relative gains, among others.
In a most preferred embodiment, the voice store and forward system includes a plurality of vocoders of different types. When a transaction for recording a message is being set, the telecommunication network derives, from a user""s profile database, the xe2x80x9cuser preferredxe2x80x9d vocoder for a particular user and transmits an identifier to the voice store and forward system. If there is no user preference, the voice store and forward system will select a default vocoder. On the basis of this identifier, the switch will assume the appropriate vocoder selection position such that the incoming audio information, in PCM format, will be converted into compressed format by a user determined or default vocoder type. Typically, the xe2x80x9cuser preferredxe2x80x9d vocoder type will be the same as the vocoder type of the user""s mobile or fixed wireless terminal. This configuration enhances speech quality.
The voice storage and forwarding system includes tagging means that attach or associate some sort of vocoder type designator to the audio data in compressed format that is stored on the storage medium. The vocoder type designator is an identifier that uniquely identifies the vocoder used for compressing the stored speech signal. This information is used to allow the system to select the proper decoder when the audio information is to be converted into decompressed format. The vocoder type designator can be stored with the individual data frames in the storage medium or it can be collected separately from the data frames.
As embodied and broadly described herein, the invention also provides a communication system, comprising:
a database storing a plurality of vocoder type identifiers;
a voice storage and forwarding device;
a data transmission pathway between said database and said voice storage and forwarding device, said voice storage and forwarding device including:
a) an input for receiving a digital signal that conveys audio information;
b) a group of vocoders;
c) a switch capable of acquiring a plurality of vocoder selection positions, in each vocoder selection position said switch directing the digital signal that conveys audio information received at said input to a selected one of said vocoders of said group of vocoders, said switch being responsive to a vocoder type identifier issued by said database and forwarded to said voice storage and forwarding device on said data transmission pathway to acquire a vocoder selection position indicative by the vocoder type identifier issued by said database;
d) a storage medium in a data communicative relationship with said vocoders, said storage medium capable to store data that conveys audio information issued by a vocoder from said group of vocoders and transmit stored data that conveys audio information toward a selected one of said vocoders.
As embodied and broadly described herein, the invention also provides a method for storing data containing audio information, said method comprising the steps of:
providing a group of vocoders;
providing a switch capable of acquiring a plurality of vocoder selection positions, in each vocoder selection position said switch directing data containing audio information to a selected one of said vocoders of said group of vocoders;
providing a storage medium in a data communicative relationship with said vocoders, said storage medium capable to store data containing audio information issued by a vocoder from said group of vocoders;
setting said switch to a selected vocoder selection position to permit transfer of data containing audio information toward a vocoder from said group of vocoders corresponding to the position of said switch.
As embodied and broadly described herein, the invention also provides a voice storage and forwarding device, comprising:
an input for receiving a digital signal that conveys audio information;
at least one vocoder;
a switch capable of acquiring at least two operative positions, namely a vocoder selection position and a bypass position, in said vocoder selection position said switch directing the digital signal that conveys audio information received at said input to said vocoder;
a storage medium in a data communicative relationship with said vocoder and with said switch, said storage medium capable to store data that conveys audio information issued by said vocoder and transmit stored data that conveys audio information toward said vocoder;
in said bypass position said switch bypassing said vocoder and directing the digital signal that conveys audio information received at said input toward said storage medium.
In a most preferred embodiment, the switch of the voice storage and forwarding system is responsive to a control message, conveyed through in-band signaling, to acquire the bypass position. Typically, such control message will be issued by another bypass capable digital signal processor with whom audio information is being exchanged. In a specific example, consider the situation where a user leaves a voice message by using a wireless terminal. During the message recording transaction, the digital signal processor at the base station or base station controller assigned to the mobile terminal will issue a control message to the switch in the voice storage and forwarding device. As mentioned earlier, this control message is sent via in-band signaling. The control information contains a portion that identifies the digital signal processor and which is intended to convey the information that in fact this digital signal processor is bypass capable. When the switch receives this control message, it extracts the identification portion and through a simple database seeking operation determines that the source of the message has a bypass capability. The switch then issues an acknowledgement message, also by using in-band signaling, that also contains an identification portion. This acknowledgement message, when received by the digital signal processor at the base station is decoded and on the basis of the identifier contained therein knowledge is derived to the effect that the source of the acknowledgement message is bypass capable. At this point, the digital signal processor in the base station or base station controller and the switch in the voice storage and forwarding device acquire the bypass mode allowing transmission of data packets without any processing by a vocoder.
As embodied and broadly described herein, the invention also comprises a communication system, comprising:
a base station or base station controller including a digital signal processor;
a voice storage and forwarding device;
a data transmission pathway between said base station and said voice storage and forwarding device, said voice storage and forwarding device including:
a) an input for receiving a digital signal that conveys audio information;
b) at least one vocoder;
c) a switch capable of acquiring at least two operative positions, namely a vocoder selection position and a bypass position, in said vocoder selection position said switch directing the digital signal that conveys audio information received at said input to said vocoder;
d) a storage medium in a data communicative relationship with said vocoder and with said switch, said storage medium capable to store data that conveys audio information issued by said vocoder and transmit stored data that conveys audio information toward said vocoder;
e) in said bypass position said switch bypassing said vocoder and directing the digital signal that conveys audio information received at said input toward said storage medium;
f) said switch being responsive to a control message issued by said digital signal processor and forwarded to said voice storage and forwarding device on said data transmission pathway to acquire said bypass position.
As embodied and broadly described herein, the invention also provides method for storing data containing audio information, said method comprising the steps of:
providing a vocoder;
providing a switch capable of acquiring either one of a vocoder selection position and a bypass position, in said vocoder selection position said switch directing data containing audio information to said vocoder;
providing a storage medium in a data communicative relationship with said vocoder, said storage medium capable to store data containing audio information issued by said vocoder;
in said bypass position said switch bypassing said vocoder and directing the data containing audio information toward said storage medium;
setting said switch to a selected one of a vocoder selection position and bypass position to permit transfer of data containing audio information to said storage device.